Polyethercyclicpolyol (PECP) has been tested as an additive for aqueous drilling muds. In this test, the performance of PECP was evaluated and found to match or exceed expectations with regard to those characteristics which were used in optimizing its molecular structure and properties. The five performance criteria that were used to guide and optimize synthesis efforts were (1) fluid loss inhibition, (2) inhibition of shale swelling, (3) inhibition of cuttings dispersion, (4) thickness of filter cake, and (5) toxicity. The performance of PECP base muds in the field test was superior in all five aspects for which experimental testing had been conducted. However, it was found that in the tests, drilling muds containing 5-10% by weight PECP tend to adhere to stabilizer surfaces. Stabilizers are large, metallic vanes, 8-10 ft long, attached radially from the drilling shaft. The tendency of PECP-containing muds to adhere to the stabilizers causes frictional resistance to drilling and slows the rate of penetration. This effect also results in "bit-bailing," i.e., formation of masses of drilling-mud containing the cuttings which stick to the drilling bit.